This invention relates to polyolefin tapes, and more particularly to the methods used in preparing isotactic polypropylene tapes to effect improvements thereof.
The polymers normally employed in the preparation of polypropylene oriented films and tapes are isotactic polymers, although on some occasions the use of syndiotactic polymers has been proposed. Isotactic polypropylene is one of a number of crystalline polymers that can be characterized in terms of the stereoregularity of the polymer chain. Various stereo-specific structural relationships denominated primarily in terms of syndiotacticity and isotacticity may be involved in the formation of stereoregular polymers of various monomers.
Isotactic polypropylene is conventionally used in the production of relatively thin tapes in which the polypropylene is heated and then extruded through dies as a sheet or film that is cut into tape segments and then subjected to orientation by stressing the tape in a longitudinal direction (referred to as the machine direction). The structure of isotactic polypropylene is characterized in terms of the methyl group attached to the tertiary carbon atoms of the successive propylene monomer units lying on the same side of the main chain of the polymer. That is, the methyl groups are characterized as being all above or below the polymer chain. Isotactic polypropylene can be illustrated by the following chemical formula: 
Another way of describing the structure is through the use of NMR. Bovey""s NMR nomenclature for an isotactic pentad is . . . mmmm . . . with each xe2x80x9cmxe2x80x9d representing a xe2x80x9cmesoxe2x80x9d dyad, of successive methyl groups on the same side of the plane of the polymer chain. As is known in the art, any deviation or inversion in the structure of the chain lowers the degree of isotacticity and crystallinity of the polymer.
Isotactic polymers are semi-crystalline and are insoluble in xylene. This crystallinity distinguishes isotactic polymers from an atactic polymer, which is non-crystalline and highly soluble in xylene. An atactic polymer exhibits no regular order of repeating unit configurations in the polymer chain and forms essentially a waxy product. In most cases, the preferred polymer configuration for oriented propylene films and tapes will be a predominantly isotactic or syndiotactic polymer with very little atactic polymer.
The isotactic polymers normally employed in the preparation of oriented polypropylene films are usually those prepared through the use of conventional Ziegler-Natta catalysts of the type disclosed, for example, in U.S. Pat. Nos. 4,298,718 and 4,544,717, both to Myer et al. U.S. Pat. No. 5,573,723 to Peiffer et al discloses a process for producing biaxially-oriented polypropylene film based on an isotactic polypropylene homopolymer or propylene/ethylene co-polymers produced by catalysis with such conventional Ziegler-Natta catalysts. Other co-polymers of propylene and alpha-olefins having from 4-8 carbon atoms also may be employed in the Peiffer process.
Catalysts employed in the polymerization of alpha-olefins may be characterized as supported catalysts or unsupported catalysts, sometimes referred to as homogeneous catalysts. Traditional supported catalysts are the so-called xe2x80x9cstandardxe2x80x9d Ziegler-Natta catalysts, such as titanium tetrachloride supported on an active magnesium dichloride as disclosed, for example, in the aforementioned patents to Myer et al. A supported catalyst component, as disclosed in the Myer ""718 patent, includes titanium tetrachloride supported on an xe2x80x9cactivexe2x80x9d anhydrous magnesium dihalide, such as magnesium dichloride or magnesium dibromide. The supported catalyst component in Myer ""718 is employed in conjunction with a co-catalyst such as an alkylaluminum compound, for example, triethylaluminum (TEAL). The Myer ""717 patent discloses a similar compound that may also incorporate an electron donor compound that may take the form of various amines, phosphenes, esters, aldehydes, and alcohols. Metallocene catalysts are often employed as unsupported or homogeneous catalysts, although, as described below, they also may be employed in supported catalyst components.
Alternative types of catalysts that produce isotactic polyolefins are disclosed in U.S. Pat. Nos. 4,794,096 and 4,975,403. These patents disclose chiral, stereorigid metallocene catalysts that polymerize olefins to form isotactic polymers and are especially useful in the polymerization of highly isotactic polypropylene. As disclosed, for example, in the aforementioned U.S. Pat. No. 4,794,096, stereorigidity in a metallocene ligand is imparted by means of a structural bridge extending between cyclopentadienyl groups. Specifically disclosed in this patent are stereoregular hafnium metallocenes which may be characterized by the following formula:
Rxe2x80x3(C5(Rxe2x80x2)4)2HfQpxe2x80x83xe2x80x83(2) 
In Formula (2), (C5(Rxe2x80x2)4) is a cyclopentadienyl or substituted cyclopentadienyl group, Rxe2x80x2 is independently hydrogen or a hydrocarbyl radical having 1-20 carbon atoms, and Rxe2x80x3 is a structural bridge extending between the cyclopentadienyl rings. Q is a halogen or a hydrocarbon radical, such as an alkyl, aryl, alkenyl, alkylaryl, or arylalkyl, having 1-20 carbon atoms and p is 2.
The various metallocene structures as described above can be used either as so-called xe2x80x9cneutral metallocenesxe2x80x9d in which case an alumoxane, such as methylalumoxane, is used as a co-catalyst, or they can be employed as so-called xe2x80x9ccationic metallocenesxe2x80x9d which incorporate a stable non-coordinating anion and normally do not require the use of an alumoxane. For example, syndiospecific cationic metallocenes are disclosed in U.S. Pat. No. 5,243,002 to Razavi. As disclosed there, the metallocene cation is characterized by the cationic metallocene ligand having sterically dissimilar ring structures that are joined to a positively-charged coordinating transition metal atom. The metallocene cation is associated with a stable non-coordinating counter-anion. Similar relationships can be established for isospecific metallocenes.
While metallocene catalysts are generally proposed for use as homogeneous catalysts, it is also known in the art to provide supported metallocene catalysts. As disclosed in U.S. Pat. Nos. 4,701,432 and 4,808,561, both to Welborn, a metallocene catalyst component may be employed in the form of a supported catalyst. As described in the Welborn ""432 patent, the support may be any support such as talc, an inorganic oxide, or a resinous support material such as a polyolefin. Specific inorganic oxides include silica and alumina, used alone or in combination with other inorganic oxides such as magnesia, zirconia, and the like. Non-metallocene transition metal compounds, such as titanium tetrachloride, are also incorporated into the supported catalyst component. The Welborn ""561 patent discloses a heterogeneous catalyst that is formed by the reaction of a metallocene and an alumoxane in combination with the support material. A catalyst system embodying both a homogeneous metallocene component and a heterogeneous component, which may be a standard supported Ziegler-Natta catalyst, e.g. a supported titanium tetrachloride, is disclosed in U.S. Pat. No. 5,242,876 to Shamsoum et al. Various other catalyst systems involving supported metallocene catalysts are disclosed in U.S. Pat. No. 5,308,811 to Suga et al and No. 5,444,134 to Matsumoto.
A polymer slit film or tape is formed by providing an isotactic propylene polymer prepared by the polymerization of propylene in the presence of an isospecific metallocene catalyst to obtain an isotactice polypropylene. The isotactic polypropylene is heated, extruded in an extruder and withdrawn from the extruder to form a sheet. Pressures within the extruder may be less than about 5000 psi, with a pressure between 2000 to 3500 psi being preferred. At least one tape segment is formed from the sheet. The tape segment is then drawn longitudinally to a draw ratio of at least about 4.5:1. During drawing, the isotactic polypropylene may be maintained at a temperature of between about 250 to 450xc2x0 F.
It has been found that polypropylene polymer tapes produced with metallocene catalyst that are drawn at draw ratios above 4.5:1, in accordance with the present invention, provide a greater percentage elongation at break and a higher tenacity at maximum elongation than do those polypropylene polymer tapes produced using conventional Ziegler-Natta catalyst for the same given draw ratios. Drawing the isotactic polypropylene at these draw ratios has resulted in polypropylene tapes having a tenacity at maximum elongation of at least about 6 g/denier and an elongation at break of greater than about 12.0%.